PORTLAND, Ore. — They huddled against the biting wind, pacing from one corner to another hoping to score heroin or pills. But a different drug was far more likely to be on offer outside the train station downtown, where homeless drug users live in tents pitched on the sidewalk.

“Everybody has meth around here — everybody,” said Sean, a 27-year-old heroin user who hangs out downtown and gave only his first name. “It’s the easiest to find.”

The scourge of crystal meth, with its exploding labs and ruinous effect on teeth and skin, has been all but forgotten amid national concern over the opioid crisis. But 12 years after Congress took aggressive action to curtail it, meth has returned with a vengeance. Here in Oregon, meth-related deaths vastly outnumber those from heroin. At the United States border, agents are seizing 10 to 20 times the amounts they did a decade ago. Methamphetamine, experts say, has never been purer, cheaper or more lethal.

Oregon took a hard line against meth in 2006, when it began requiring a doctor’s prescription to buy the nasal decongestant used to make it. “It was like someone turned off a switch,” said J.R. Ujifusa, a senior prosecutor in Multnomah County, which includes Portland.

“But where there is a void,” he added, “someone fills it.”

The decades-long effort to fight methamphetamine is a tale with two takeaways. One: The number of domestic meth labs has declined precipitously, and along with it the number of children harmed and police officers sickened by exposure to dangerous chemicals. But also, two: There is more meth on the streets today, more people are using it, and more of them are dying.

Drugs go through cycles — in the 1980s and early ’90s, the use of crack cocaine surged. In the early 2000s, meth made from pseudoephedrine, the decongestant in drugstore products like Sudafed, poured out of domestic labs like those in the early seasons of the hit television show “Breaking Bad.”

Narcotics squads became glorified hazmat teams, spending entire shifts on cleanup. In 2004, the Portland police responded to 114 meth houses. “We rolled from meth lab to meth lab,” said Sgt. Jan M. Kubic of the county sheriff’s office. “Patrol would roll up on a domestic violence call, and there’d be a lab in the kitchen. Everything would come to a screeching halt.”

In 2005 Congress passed the Combat Methamphetamine Act, which put pseudoephedrine behind the counter, limited sales to 7.5 grams per customer in a 30-day period and required pharmacies to track sales. Although some meth makers tried “smurfing,” sending emissaries to several stores to make purchases, meth cases plummeted.

States like Oregon and Mississippi required a prescription, making smurfing almost impossible. And a new epidemic took hold: prescription painkillers and opiates like heroin. With no more meth lab explosions on the nightly news, the public forgot about the drug.

But meth, it turns out, was only on hiatus. When the ingredients became difficult to come by in the United States, Mexican drug cartels stepped in. Now fighting meth often means seizing large quantities of ready-made product in highway stops.

The cartels have inundated the market with so much pure, low-cost meth that dealers have more of it than they know what to do with. Under pressure from traffickers to unload large quantities, law enforcement officials say, dealers are even offering meth to customers on credit. In Portland, the drug has made inroads in black neighborhoods, something experienced narcotics investigators say was unheard-of five years ago.

“I have been involved with meth for the last 25 years. A wholesale plummet of price per pound, combined with a huge increase of purity, tells me they have perfected the production or manufacturing of methamphetamine,” said Steven Bell, a spokesman for the Drug Enforcement Administration. “They have figured out the chemical reactions to get the best bang for their bucks.”

Nearly 100 percent pure and about $5 a hit, the new meth is all the more difficult for users to resist. “We’re seeing a lot of longtime addicts who used crack cocaine switch to meth,” said Branden Combs, a Portland officer assigned to the street crimes unit. “You ask them about it, and they’ll say: ‘Hey, it’s half the price, and it’s good quality.’”

Nationally, nearly 6,000 people died from stimulant use — mostly meth — in 2015, a 255 percent increase from 2005, according to the Centers for Disease Control and Prevention. The percentage of the nation’s drug overdose toll that was attributed to stimulants inched up to 11 percent of the deaths.

In Oregon, 232 people died from meth use in 2016, nearly twice as many as died from heroin — and three times as many as died from meth 10 years before, according to the state Department of Health.

Between 2011 and 2015, meth arrests were the only type of drug arrests in Portland to increase, and meth has the highest correlation with serious crimes. More than one in five burglars and nearly 40 percent of car thieves were also charged with meth crimes, according to the Portland Police Bureau.

“Heroin is a depressant. It shuts you down and you’re not capable of doing a whole lot,” Sergeant Kubic said. Meth is a stimulant: “Tweakers are jacked up. They have lowered inhibitions and are awake 24/7, running around at night, so burglaries become easier.”

Eric, a former dancer in Portland, who asked that his last name not be published, said he now worked as a “professional booster.” Pawnshop owners give him “laundry lists” of coveted items, and he goes out and steals them, getting 50 cents on the dollar.

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A person injects a combination of methamphetamine and heroin in Portland. Meth arrests in Oregon rose 64 percent from 2011 to 2015, the only drug-related arrest category to increase.Credit...Amanda Lucier for The New York Times

The cartels’ efficiency has flooded the market far beyond Oregon. In 2016, customs authorities in San Diego seized 21,747 pounds of meth, almost 10 times what was apprehended in 2007. At border points in Arizona, California and Texas, agents seized 24 times as much.

In Montana, meth violations more than tripled between 2010 and 2015. In drug-related deaths in Oklahoma, meth is by far the No. 1 cause (oxycodone was a distant second). In Hawaii, where meth was first introduced in the United States, the number of people over 50 who said meth was their drug of choice has doubled in five years. In South Dakota, the attorney general has proclaimed an epidemic.

To counteract the falling price, drug cartels are actively pursuing new markets on the East Coast, according to the National Drug Threat Assessment released by the D.E.A. last fall.

Meth is carried across the border by people on foot, or hidden in cars and trucks. It can be converted to liquid, and has been smuggled in iced tea bottles, disguised as horse shampoo and hidden in tortillas.

The ingredient initially used to make it, ephedrine, was first synthesized in 1887 and later used to treat asthma. It was often used in the military and by truckers who needed to stay awake. For decades, United States lawmakers have been trying to curtail its use. But each time an ingredient was outlawed, something else took its place.

In 2007, Mexico cracked down on pseudoephedrine. The cartels reverted to using phenyl-2-propanone, known as P2P, a method popularized by biker gangs in the 1970s. Although it, too, is restricted and monitored, there are many ways to manufacture it.

Public health experts say little is being done to combat the surge in meth because it has been so overshadowed by opiates. And, there are fewer tools to combat meth than to combat opioids: There is nothing like Naloxone, which can reverse opioid overdoses, or methadone, which can stem opioid cravings.

Dr. Paul F. Lewis, the public health officer for the Portland metropolitan area, said the problem was complicated by the fact that many users take both drugs.

“We need to think about substance abuse much more broadly,” Dr. Lewis said. “Eighty or 90 percent of heroin users are also using meth. It deserves more attention.”

Eric, the former dancer, said the meth on the street was so strong that it worked best when used in combination with heroin to temper the effects.

“I have seen meth being used by people from age 12 to 90,” he said. “It’s a rush from all hell. Explosive. Intense. That goes away, and you’re up for a long period of time. At first it’s good. And then, after a while, it’s too much.”